Documentation for CollectionProperty (CRUD methods)
You are right about the documentation. The Python Console is your friend here. It offers autocomplete "intellisense" when you hit CtrlSpace after a . or a (:



You get the methods documentation from the source if there is one. In CollectionProperty case there is none:



The traceback error gives another hint:



If this does not help you search google, this site, blenderartists or your favorite blender python blogs for the functions you are interested in for more info or code examples. Since you now know what is available and what you are after, this should give better results than searching just for documentation.

If that still fails, chances are that you are after something only couple of people in the world know. You can look into the source code where you find bpy_props.c and then you dig. If you don't get too far the best bet in this case is to contact developers on official channels (mail, IRC) and ask there.